gd: explain why imagedestroy() is NOP (#4395)

Co-authored-by: Kamil Tekiela <[email protected]>

Author: pereorga

reference/image/functions/imagedestroy.xml

@@ -13,8 +13,10 @@
   </methodsynopsis>
   &note.resource-migration-8.0-dead-function;
   <para>
-   Prior to PHP 8.0.0, <function>imagedestroy</function> freed any memory associated
-   with image <parameter>image</parameter>.
+   Prior to PHP 8.0.0, <function>imagedestroy</function> freed any memory
+   associated with the <parameter>image</parameter> resource. As of 8.0.0,
+   the GD extension uses objects instead of resources, and objects
+   cannot be explicitly closed.
   </para>
  </refsect1>
  <refsect1 role="parameters">